1. Field of the Invention
The present invention relates to an electrode for a fuel cell and a manufacturing method therefor.
2. Description of the Related Art
A solid polymer electrolyte fuel cell is an apparatus comprising membrane-electrode assembly, which comprises electrodes, an anode and a cathode each having a catalyst layer and a gas diffusion layer containing an electron-conductive porous substrate, bonded onto ion-exchange membrane. Hydrogen is supplied to the anode and oxygen is supplied to the cathode so as to generate electric power by using electrochemical reactions. The electro-chemical reaction which occurs in each electrode is as follows:
Anode: H2→2H++2e
Cathode: 1/2O2+2H++2e→H2O
Overall Reaction: H2+1/2O2→H2O
As can be understood from the reaction formula, reactions in each electrode proceed in only the three-phase boundary sites in which supply and receipt of reactant gases (hydrogen or oxygen), protons (H+) and electrons (e) can simultaneously be performed.
As shown in FIG. 8, the electrode of a fuel cell incorporates a porous catalyst layer 86 in which catalyst particles 81 and solid polymer electrolyte 82 are distributed three-dimensionally with a plurality of small pores 84 are formed; and a gas diffusion layer 88 containing a electro-conductive porous substrate 87.
The gas diffusion layer 88 provides spaces in the surface of the catalyst layer 86 to maintain a passage for carrying oxygen or hydrogen as a reactant supplied from the outside of the cell. Moreover, the gas diffusion layer 88 provides a passage for discharging water produced in the catalyst layer of the anode from the surface of the catalyst layer 86 to the outside of the cell system.
On the other hand, the catalyst particles 81 of the catalyst layer 86 form electron conductive channel, the solid polymer electrolyte 82 forms a proton conductive channel and the small pores 84 form supply/discharge channel for supplying oxygen or hydrogen to the inside from the surface of the catalyst layer 86 and discharging water produced in the cathode to the surface from the inside of the catalyst layer. The three channels are three-dimensionally dispersed in the catalyst layer 86 so that an infinite number of three-phase boundary sites, in which the gas, protons (H+) and electrons (e) are supplied and receipted simultaneously, are formed in the catalyst layer 86. Thus, portions for the reaction sites of the electrode is provided.
Note that reference numeral 83 shown in FIG. 8 represents PTFE (polytetrafluoroethylene) particles which impart hydrophobicity to the inside portion of the small pores 84 of the catalyst layer 86 and the surface of the small pores 84. Reference numeral 85 represents an ion-exchange membrane.
The ion-exchange membrane 85 serving as an electrolyte exhibits satisfactory proton conductivity in a water-retention state. Therefore, the operation must be performed while a wet state is being maintained in the cell. Therefore, humidified hydrogen or oxygen are supplied to the anode or the cathode to prevent dry up of the ion-exchange membrane 85 so that water content in the ion-exchange membrane is controlled.
The solid polymer electrolyte fuel cell has the catalyst layer with the pores forming three-dimensional channel for supplying oxygen or hydrogen. Therefore, the humidified supply gas, which is the reactant, causes water to be accumulated in the surface of the catalyst layer. As an alternative to this, accumulation of water in the pores inhibits supply of the reactant gases to the three-phase boundary sites of the catalyst layer, and, in particular, to the deep portion of the electrode. Thus, an actual active surface area is reduced. Therefore, the performance of the cell cannot satisfactorily be obtained. Accordingly, adequate hydrophobicity is imparted to the gas diffusion layer containing the electro-conductive porous substrate and the catalyst layer to prevent accumulation of water.
Importance of hydrophobicity to the electro-conductive porous substrate in a case of carbon paper (having a thickness of 1.5 mm) which is made by a sintering body of carbon fibers will now be described. The carbon paper is immersed in solution of PTFE suspension. Then, the carbon paper containing the PTFE particles are baked at about 350° C. for 15 minutes in a nitrogen atmosphere so that the surfaces of the carbon fibers are coated with PTFE.
On the other hand, the hydrophobicity is imparted to the catalyst layer by mixing the PTFE suspension into paste for the catalyst layer including carbon particles supporting fine particles of noble metal as catalyst, such as platinum, and the solid polymer electrolyte solution.
The hydrophobicity of both of the catalyst layer and the electro-conductive porous substrate is, however, unsatisfactory at present. Supply of hot and enough humidified gases to improve the proton conductivity of the ion-exchange membrane resulting in enhancement of cell power unsatisfactorily causes water to be water flooding accumulated in the pores of the catalyst layer and the surface of the same. As a result, supply of the reactant gases to the three-phase boundary sites of the catalyst layer, and, in particular, to deep portions of the catalyst layer is inhibited. As a result, the actual active surface area is reduced, causing a problem to arise in that the performance of the cell cannot sufficiently be obtained. In particular, because water is produced with the reactions proceeding, this accumulation of water is occurred easily in the pores of the catalyst layer in the cathode.
To improve the hydrophobicity of the catalyst layer, resulting in solution of the foregoing problem, the mixture ratio of the solution of PTFE particles dispersion to the past for the catalyst layer must be increased. However, the increase of the amount of the PTFE particles in the electrode reduces the ratio of the catalyst supporting on carbon, the solid polymer electrolyte and the pores. As a result, formation of the electron conductive channel, the proton conductive channel and the channel for supplying/discharging oxygen or hydrogen as a reactant and water which is a product is inhibited. Therefore, there arises a problem in that the power of the cell is undesirably reduced.
To furthermore improve the hydrophobicity of the electro-conductive porous substrate, the amount of PTFE suspension which is applied to the electro-conductive porous substrate must be increased. If the amount of it is enlarged excessively, PTFE particles close pores of the electro-conductive porous substrate. In the foregoing case, there arises a problem in that gas supply is inhibited.
In general, it is said that the effective thickness of the catalyst layer for the electrochemical reactions is 5 μm to 10 μm. The gas supply in the electrode cannot sufficiently be performed in the catalyst layer having a larger thickness. As a result, the gas is wasted and the function of the electro-conductive porous substrate for maintaining the passage for the gas is inhibited. Therefore, control of the thickness of the catalyst layer is an important factor to improve the performance of the electrode.
However, in general, carbon paper which is the conventional electro-conductive porous substance is manufactured by molding carbon fibers each having a diameter of 5 μm to 10 μm into an unwoven shape, followed by sintering. The average diameter of the pores of the substrate is about 10 μm to about 20 μm. Therefore, it is very difficult to keep the thickness of 5 μm to 10 μm of the catalyst layer. A cross section of a state where the conventional catalyst layer has been formed on the carbon paper gas diffusion layer is shown in FIG. 9. Reference numeral 91 represents the catalyst layer and 92 represents the carbon paper as the electro-conductive porous substance. Since the diameter of the pores in the electrochemical reactions made of the porous material is large and its pores are roughly distributed, the thickness of the applied catalyst layer 91 is ununiform.
To prevent the foregoing problem, the porosity and its pore diameter of the porous substance must be reduced and a dense structure must be formed. In the foregoing case, the passage for the gas cannot easily be maintained.